The invention relates to a method for producing titanium-doped synthetic quartz glass.
Producing synthetic quartz glass for commercial applications, it is customary to generate SiO2 particles from a silicon-containing starting substance in a CVD (chemical vapor deposition) procedure by means of hydrolysis and/or oxidation followed by precipitation of these on a support. The method can be subdivided into external and internal deposition methods. In external deposition methods, SiO2 particles are applied onto the outside of a rotating support. Examples to be mentioned include the OVD method (outside vapor phase deposition), VAD method (vapor phase axial deposition) or the PECVD method (plasma-enhanced chemical vapor deposition). The most prominent example of an internal deposition method is the MCVD method (modified chemical vapor deposition), in which SiO2 particles are deposited on the internal wall of a tube that is heated from outside. It shall be noted in this context that the simplified chemical notation of chemical compounds (e.g. “SiO2”) and the molecular formula (e.g. “SiO2”) are considered to be equal and are used interchangeably herein.
The reflecting optical systems and masks used In EUV lithography (extreme ultraviolet) require substrate materials that show very little to no noticeable thermal expansion in the temperature range of 20° C. to 50° C. This requirement can be met through titanium-doped quartz glass. A method for producing such titanium-doped synthetic quartz glass (often referred to as ULE glass) is described in U.S. Pat. No. 5,970,751 A.
Printed patent specification U.S. Pat. No. 5,970,751 A describes the use of polyalkylsiloxane (also referred to as “siloxane” for short) for producing titanium dioxide-doped quartz glass. In the description of the invention hereinafter, titanium dioxide-doped quartz glass is also referred to as titanium-doped quartz glass.
Polyalkylsiloxanes are characterised by having a particularly high fraction of silicon per weight fraction, which contributes to the economic efficiency of their use in the production of synthetic quartz glass. The substance group of siloxanes can be subdivided into open-chain polyalkylsiloxanes and closed-chain polyalkylsiloxanes. The general molecular formula of polyalkylsiloxanes is SipOp(R)2P, with P being an integer ≧2. The residue “R” is an alkyl group, i.e. a methyl group in the simplest case. Due to the availability of commercial quantities at high purity, octamethylcyclotetrasiloxane (OMCTS) is currently used preferably. According to a notation introduced by General Electric Inc., said substance is also referred to as “D4”, whereby “D” represents the [(CH3)2Si]-0-group.
However, the production of pure D4 necessitates a time-consuming and costly distillation procedure due to the relatively high boiling temperature and the chemical similarity to other polyalkylcyclosiloxanes such as hexamethylcyclotrisiloxane (D3), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and tetradecamethylcycloheptasiloxane (D7). Please refer to U.S. Pat. No. 5,879,649 A in this context.
The production methods described in the prior art lead to inhomogeneities in the materials in large-volume cylinder-shaped soot bodies with external diameters of more than 300 mm, for example in the form of axial, radial or azimuthal variations of the hydroxy groups or titanium concentration or viscosity values.